The usual method for quantitatively determining water is the Karl Fischer method in which the substance to be analyzed is reacted with sulfur dioxide and iodine dissolved in a mixture of pyridine and methanol; see K. Fischer, Angew. Chemie, vol. 48 (1935), page 394. The reagent reacts with water to give pyridine sulfate and hydrogen iodine. In this process the reagent undergoes decoloration. The iodine consumption is a measure for the water content of the substance. The reaction proceeds according to the following equation: EQU SO.sub.2 +I.sub.2 +2H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +2HI
The titrimetric determination is very accurate. The reagent allows a water content of less than 0.01% to be detected; see Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition, vol. 2 (1963), pages 673-677.
A disadvantage of the Karl Fischer method is the fact that the reaction proceeds slowly, that titration is therefore laborious and time consuming and that the end-point is distorted. An inconvenience is the annoying odor caused by the sulfur dioxide andpyridine. Moreover pyridine makes it necessary to perform the process under a fume hood. Another disadvantage is the yellow SO.sub.2 I.sup.- complex formed by the sulfur dioxide and iodide which excludes the visual determination of the end point.
The limited life, the unstable titer and the necessity of storage in the dark and under cool conditions are further drawbacks.
The limited possibilities of the use and the not very stable titration conditions are further problems which the analyst faces despite the fact that the Karl Fischer method has been substantially improved.
In a known further development of this Karl Fischer method the problems of the titrimetric determination of water are avoided; see J. C. Verhoef and E. Barendrecht, Analytica Chimica Acta, vol. 94 (1977), pages 395-403. This improved method makes use of two reagents, i.e. a solution of sodium acetate and sulfur dioxide in methanol (solution A) and a solution of iodine in methanol (titration solution B).
The so-called blind value of solution A is a measure of its stability. In practice it has been found that the blind value of solution A increases at 18.degree. to 20.degree. C. during a week by 0.1 ml. The reason is that the ester reaction proceeds according to the scheme: EQU alcohol+acid.fwdarw.ester+water
This reaction becomes more noticeable at higher temperatures.
In the present case the following esterification reaction takes place: EQU methanol+acetic acid.fwdarw.methyl acetate+water
Blind values of about 20 ml therefore appear especially in hot countries because of the ester reaction that takes place at the higher temperatures prevailing there, i.e. 0.4 to 0.5 ml of the solution A per month at 40.degree. C. In the second or third month the increase is 0.5 to 1 to 2 ml of the solution A.
Because in titration acid is formed, the buffer capacity plays a very important part since the iodometric water determination is a redox titration which is heavily dependent on the pH-value.
The titration procedure is as follows: 20 ml of solution A are pretitrated with the titrating solution B under steady stirring and with moisture being excluded. A specified amount of the water-containing substance to be analyzed is then quickly placed into the titration vessel. The amount of the substance to be analyzed (amount of test sample) should be adequately proportioned to the estimated amount of water present.
With the appropriate buffer capacity, it is possible to determine 100 to 110 mg of water in 20 ml of solution A. The titration vessel is closed, the buret adjusted and titration is started. During the whole titration procedure the solution is to be thoroughly mixed with a magnetic stirrer.
The bipotentiometric method is used in the most usual titrations for determining the end point. The reduction time is normally 20 seconds, at which time the point of equivalence is reached.
With this method, aquametry without interference is possible in alcohols, alkanes, aromatic hydrocarbons, aldehydes, ketones, ethers, esters, salts with crystallization water, basic substances such as tris-(hydroxymethyl)-amino methane, lyophilized products, food, molecular sieves and granular fertilizers. This method also lends itself to the visual determination of the end point.
Thus the problem underlying the present invention is to develop a reagent for quantitatively determining water which is still stable even at elevated temperatures, e.g. 40.degree. C., which reagent is based on the above-described known reagent solution A which is used together with the titrating solution B, and which when allowed to stand does not show an increase of the blind value even at elevated temperatures and results in minimum precipitations.